The present invention relates to a method of detecting residual pressure in a braking system, particularly in a brake cylinder.
In the case of conventional braking systems, residual pressure cannot be recognized by the braking system. However, even slight residual pressure may cause an application of the brake without creating a noticeable braking effect for the driver. An applied or “grinding” brake may cause overheating and, in the worst case, may result in a complete failure of the brake. As a rule, the driver can recognize a defect only in the case of a noticeable braking effect, which frequently has already been preceded by severe damage to the brake.
In the case of an electronically controlled pneumatic brake (EBS) for commercial vehicles, a desired pressure value is controlled into a brake cylinder, while a pressure regulating module is connected to the input side. This takes place in that a control pressure is controlled into a control chamber of the pressure regulating module by the use of two solenoid valves. By the use of an intake valve, the control chamber can be acted upon by storage pressure, which leads to a ventilation of the control chamber. By the use of the outlet valve, the control chamber can be connected with the atmosphere, that is, with the environment, in order to achieve a bleeding of the control chamber and, thus, a release of the brake. By the use of a relay piston, a pressure is controlled into the working chamber, which corresponds approximately to the pressure in the control chamber. The working chamber is connected with the brake cylinder. By the opening of a third solenoid valve, which is called a back-up valve, the control chamber can be connected with brake pressure of a conventional braking system, which is present for back-up purposes. The actual pressure in the working chamber is sensed via a pressure sensor. The electronic braking system monitors a possibly existing deviation between the actual-pressure signal and the defined desired pressure and correspondingly controls the intake valve or the outlet valve. In the unbraked condition, the intake valve and the outlet valve are closed and the back-up valve is opened.
As a result of diverse disturbing influences or defects in the braking system, a residual pressure may remain or be created in the braking system even if the driver does not operate the brake pedal. The residual pressure may be caused, for example, by a leaky intake valve, whereby a ram pressure is built up in the control chamber, which ram pressure cannot be completely bled by way of the back-up valve. Another cause may be a defect of the conventional braking system present “as a back-up”, in which case, a pressure can then be built up in the control chamber, as required, by way of the back-up valve.
It is an object of the invention to provide a method and a device, respectively, for detecting residual pressure in a braking system, particularly in the working chamber of a brake cylinder.
This object is achieved by providing a method and device for detecting residual pressure in a braking system, particularly in a brake cylinder, characterized by (a) determining a suspicion of residual pressure when a difference between an adaptive pressure signal, whose time slope is caused to qualitatively follow a sensed actual pressure course, and a previously measured actual pressure (UB0, UB1) exceeds a defined limit value (UA−UB0); and (b) checking the suspicion of a residual pressure by operating a pressure relief element of the braking system and monitoring a possibly occurring drop of the sensed actual pressure. Advantageous embodiments and further developments of the invention are described herein.
The basic principle of the invention consists of determining by means of a continuous “rough” monitoring of the pressure in the braking system whether there is a suspicion of a residual pressure. If such a residual-pressure suspicion was determined, a precision monitoring takes place, as required, in a manner that is repeated several times. In this case, a pressure relief element of the braking system is opened up and sensed in order to determine whether the actual pressure is decreasing and, if so, to what extent.
This basic differentiation between a rough monitoring and a precision monitoring is required because the output signal of the pressure sensor alone is not sufficient for reliably detecting a residual pressure, that is, a defect in the braking system when a so-called “adaptive offset compensation” takes place. When the brake is not operated, that is, not released, the pressure in the braking system or the output signal of the pressure sensor depends on various influencing variables, such as the ambient temperature, the altitude or the atmospheric pressure. In addition, as a result of certain influences, a vacuum may sometimes temporarily be generated in the working chamber.
The “zero pressure level” (offset) of an intact braking system may therefore fluctuate within a certain bandwidth as a result of the above-mentioned disturbing influences. An “offset compensation” therefore has to be carried out; that is, the zero pressure level has to be adapted to the respective ambient condition.
By operating the outlet valve and analyzing (filtering) the pressure sensor voltage, a residual pressure detection can therefore be achieved. At a point in time, at which there is no residual pressure, a pressure sensor voltage is stored as a reference voltage. For this purpose, the outlet valve is opened in an initialization phase. During the operation, the pressure sensor voltage, that is, the pressure level existing in the braking system, may change as a function of different influences. As a result of a constant analysis of the (filtered) course of the pressure sensor voltage and a comparison with the reference voltage, which corresponds to a condition without residual pressure, a suspicion of a residual pressure can be determined. By the use of a further operation of the outlet valve, it can be detected whether an excess pressure actually exists in the braking system; that is, whether a residual pressure can be bled. If no bleeding takes place when the outlet valve is operated again, it is assumed that the zero pressure level has shifted, and the reference voltage is adapted correspondingly. If, in contrast, in the case of an operation of the outlet valve taking place possibly several times successively, a residual pressure bleeding takes place in every case, then a disturbance in the braking system can be assumed.
Practically, the following steps are carried out when the residual pressure monitoring is implemented:
Sensing of the actual pressure in the braking system and generating of a corresponding actual pressure signal;
during an initialization phase,
opening of the pressure relief element,
closing of the pressure relief element and storage of the actual pressure measured during the closing as a reference pressure signal;
generating an adaptive pressure signal whose signal course is formed by the constant filtering of the course of the actual-pressure signal, in which case, during the closing of the pressure relief element, the adaptive pressure signal is set to an initial value which is equal to that of the actual pressure signal;
monitoring of the difference between the reference pressure signal and the adaptive pressure signal and another opening of the pressure relief element when the difference is greater than a defined limit value;
storing of the actual pressure measured during the repeated opening as a starting pressure signal;
determining that
no residual pressure is present when the following applies after the repeated opening:actual pressure signal>starting pressure signal−limit value
or
a residual pressure is present when the following applies after another opening:actual pressure signal<starting pressure signal−limit value.
The adaptive pressure signal is formed by filtering the actual pressure signal. The course of the adaptive pressure signal corresponds qualitatively, that is, “roughly”, to the actual pressure signal, in which case, for example, when the actual pressure signal rises, the adaptive pressure signal also rises and vice-versa. In this case, one or several functional or relational “filtering rules” may be defined.
When, during the rough monitoring, the adaptive pressure signal and the reference pressure signal diverge and a defined difference (limit value) is exceeded, it is suspected that a residual pressure has formed in the braking system. When there is a suspicion of a residual pressure, a precision monitoring is carried out.
For this purpose, an “outlet valve” of the braking system is actuated, so that a possibly existing excess pressure can escape. If no “noticeable” pressure drop takes place, it is assumed that the braking system is in order and only environmental conditions, such as the temperature, the atmospheric pressure, etc., have changed. Therefore, when the bleeder valve is closed, the reference pressure level is changed corresponding to the “new normal conditions”; that is, the reference pressure level is set to the level of the actually measured actual pressure. Also, the “new initial value” for the adaptive pressure signal is set to this pressure level.
If, in contrast, an excess pressure is present in the braking system, which escapes when the bleeder valve is opened, and the measured actual value drops by more than the defined limit value, the reference pressure signal is retained unchanged, and the “old” reference pressure level is also set as the new initial value for the adaptive pressure signal. If such a residual pressure bleeding is determined several times successively, it can be assumed with relatively high certainty that a disturbance of the braking system is actually present, and a warning signal is generated.
Among others, the following advantages are achieved by the use of the invention:
Even a slight residual pressure can be detected;
The residual pressure monitoring takes place permanently during an unbraked drive, whereby a forming residual pressure can be detected before the brake overheats;
A detection of residual pressure can be achieved in all cases; that is, in the case of an abrupt formation of residual pressure as well as in the case of a creeping formation of residual pressure over an extended period of time;
The additional stress to the bleeding valve is very low because the number of additional operations is low; and
The residual pressure monitoring can be used for aiding the pressure sensor offset compensation.
The invention has a very broad usage range and can generally be used when:
a magnitude of a technical system depends on different influencing variables;
it should be detectable at any time whether a change of the magnitude is caused by a certain influencing variable;
the possibility exists of acting upon a magnitude and detecting by the effect whether the preceding change of the magnitude was caused by a certain influencing variable;
the possibility of influencing the magnitude is to be used only rarely or exists only rarely.
In the following, the invention will be explained in detail by means of an embodiment in connection with the drawing.